﻿6 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



HEAT IN THE KILN. 



Hoat is used in a kiln to produce rapid evaporation and to hasten 

 tlie transfusion of moisture from the interior to the surface of the 

 wood. The correct temperature to use is determined by the char- 

 acter of the wood and varies widely with different kinds of stock. 

 Commercial kiln temperatures range from 100° to 250° F. 



The use of temperatures above that of the surrounding atmosphere 

 introduces a problem in the heating of buildings, and imposes an 

 added burden upon the heating system, namely, to keep the kiln 

 building hot and to replace the heat lost through the walls of the 

 kiln. The higher the kiln temperature, the greater will be these 

 heat losses. The amount of heat actually used in the evaporation 

 of the moisture is only a small part of the total heat supplied; it 

 is seldom over 40 per cent and frequently as low as 5 per cent, 

 depending upon the kind of drying being done. 



SOURCES OF HEAT. 



Many methods have been used to heat kilns, and although most 

 of them are obsolete or impractical, brief mention will be made of 

 the principal ones. 



Direct furnace heat. — Smoke and other products of combustion 

 are led direct from an ordinary furnace into the kiln, from which 

 they are exhausted by chimney or other suitable means. Kilns of 

 this type are known as " smoke kilns." At one time it was thought 

 that lumber dried in them was superior to steam-dried stock, but 

 their use has been largely abandoned. 



Indirect furnace heat. — As in an ordinary hot-air furnace, the 

 air passes around the fire pot and radiators on its way to the kiln, 

 and the products of combustion pass directly up the chimney instead 

 of through the kiln. 



Gas. — Occasionally natural or artificial gas is used to heat small 

 dry kilns, the burners being arranged much as in an ordinary house- 

 hold gas oven. 



Electricity. — Electric heat can be used in small kilns, although 

 the cost of current is prohibitive, except possibly for experimental 

 units. 



Hot water. — Hot-water heat can readily be adapted to the heating 

 of kilns which do not demand too high a temperature. A suitable 

 hot-water supply would rarely be available, however, in the absence 

 of steam. 



Steam. — At present steam is almost universally used for heating 

 dry kilns of all types, and a knowledge of its use is essential to 

 intelligent kiln operation. It may be either high pressure, above 10 

 pounds per square inch, or low pressure, below 10 pounds. High- 

 pressure steam is almost invariably live steam — that is, steam direct 

 from the boilers: low-pressure steam is frequently exhaust steam, 

 or that which has passed through engine, pump, or turbine on its 

 way from the boilers to the kilns. High-pressure steam is much 

 drier, as a rule, than low-pressure steam, principally because exhaust 

 steam generally carries with it much water condensed in its passage 

 through the engine or other unit in which it has done work. As the 

 steam circulates through the kiln radiators the kiln air is heated 

 and the contained lumber is dried accordingly. 



